The present invention relates to a beam directing device and a charged particle beam apparatus utilizing the same for inspecting samples by irradiating them with a focused beam of electrically charged particles, such as electrons, positrons, or ions. Such apparatus is used in the form of a scanning electron microscope (SEM), particularly for inspection of semiconductor wafers.
Scanning electron microscopy is a known technique widely used in the manufacture of semiconductor wafers, being utilized in a Critical Dimension metrology tool, the so-called CD-SEM (critical dimension scanning electron microscope) and a defect review metrology tool SEM, the so-called DR-SEM (defect review scanning electron microscope). In a SEM, the surface region of a sample to be examined is two-dimensionally scanned by means of a primary beam of electrically charged particles, usually electrons, which travel along an optical axis of the apparatus. Irradiation of the sample with the primary electron beam releases secondary (or backscattered) electrons thereby defining a secondary electron beam. The secondary electrons are released at that side of the sample at which the primary electron beam is incident, and move back to be captured by a detector, which generates an output electric signal proportional to the so-detected secondary electron beam. The energy and/or the energy distribution of the secondary electrons is indicative of the nature and composition of the sample.
An SEM typically includes such main constructional parts as an electron beam source, an electron beam column, and a detection unit. The electron beam column comprises inter alia a beam aligning means (the so-called xe2x80x9calignment coilsxe2x80x9d) and a beam shaping means (stigmator) arranged along an anode tube that defines a primary beam drift space, and comprises a focusing means for directing a primary electron beam onto a sample and directing secondary electrons towards one or more detection units. The focusing assembly typically includes an objective lens arrangement and scanning coils.
To increase the image resolution and improve image acquisition, the primary electron beam should be affected as little as possible, and secondary electrons should be completely detected. The increase of the image resolution can be achieved by reducing chromatic aberration of focusing and deflection. WO 01/45136 assigned to the assignee of the present application discloses a deflection and focusing technique, wherein chromatic aberrations are compensated by using one or two deflections of the primary electron beam propagating towards the sample, i.e., the pre-lens deflection, in-lens deflection, or both. The complete detection of the secondary electrons requires spatial separation between the primary and secondary electrons and the effective detection of the secondary electrons (with minimal losses of electrons).
In many cases, the detector is accommodated above the objective lens outside the path of the primary beam propagation through the column. To direct secondary electrons to the detector, a generator of orthogonal electric and magnetic fields (known as Wien-filter) is used (e.g., U.S. Pat. Nos. 5,894,124; 5,900,629). To ensure detection of those secondary electrons that are not sufficiently deflected by the Wien-filter, a target or extracting electrode made of a material capable of generating a secondary electron when an electron collides therewith is additionally used. Such a target is formed with an aperture and is located such that the axis of the primary beam propagation towards the focusing means intersects with this aperture, which thereby serves as a primary beam hole.
To eliminate the use of the Wien-filter, which requires extensive care and is difficult to adjust, WO 99/26272 suggests scanning a sample with one or more angles of incidence of the primary beam. According to this technique, the primary beam is directed to run diagonally to the optical axis of the focusing assembly, and is redirected into the optical axis by a redirection unit arranged below the plane of a detector accommodated outside the primary beam path. The redirection unit also affects secondary electrons in the sense that it separates primary and secondary electrons.
The technique of the above-indicated publication WO 01/45136, assigned to the assignee of the present application, utilizes a secondary electrons"" detector formed with a primary beam hole that is located in the path of a primary electron beam propagating towards the focusing assembly. Here, a deflection system is located downstream of the detector (with respect to the direction of the primary electron beam propagation towards the sample) and operates to affect the trajectory of the primary electron beam such that the primary electron beam impinges onto a sample along an axis forming a certain angle with the sample""s surface (the so-called xe2x80x9ctilt modexe2x80x9d). This is aimed at solving another problem of the inspection systems of the kind specified associated with inspecting and/or measuring on patterned surfaces. The pattern is typically in the form of a plurality of spaced-apart grooves. To detect the existence of a foreign particle located inside a narrow groove, it is desirable to tilt the scanning beam with respect to the surface. Generally, a tilt mechanism can be implemented by mechanically tilting either the sample carrier relative to the charged particle beam column (e.g., U.S. Pat. Nos. 5,734,164; 5,894,124; 6,037,589) or the column (e.g., U.S. Pat. No. 5,329,125). The technique of WO 01/45136 achieves a tilt mechanism by affecting the trajectory of the primary electron beam using single- or double-deflection. However, the column""s configuration of WO 01/45136 while providing effective detection of secondary electrons with the above-described tilt mode of operation, will be problematic for detecting secondary electrons, especially fast electrons (the so-called HAR mode), when operating with normal incidence of the primary beam, i.e., beam incidence substantially perpendicular to the sample""s surface.
There is accordingly a need in the art to improve inspection of samples with a charged particle beam by providing a novel beam directing method and device, and a charged particle beam apparatus utilizing the same.
The main idea of the present invention consists of providing effective detection of a secondary charged particle beam with a detector, which is made with an opening and has detecting regions outside this opening, and which is accommodated in the path of the primary charged particle beam such that the primary beam propagation axis intersects with the opening, which therefore serves as a primary beam hole. This is the so-called xe2x80x9cin-column detectorxe2x80x9d. The present invention provides for affecting the trajectories of primary and secondary charged particle beams propagating through a beam directing device to cause a desired incidence of the primary charged particle beam onto a sample, and to cause propagation of the secondary beam to a region of the detector outside the primary beam hole.
The term xe2x80x9cprimary beamxe2x80x9d or xe2x80x9cprimary charged particle beamxe2x80x9d used herein signifies a charged particle beam, which is formed by charged particles generated by a source (cathode), and which is to be directed to a sample to knock out charged particles forming a xe2x80x9csecondary beamxe2x80x9d (also referred to as xe2x80x9csecondary charged particle beamxe2x80x9d, which is to be detected.
The above is implemented by deflecting the primary beam entering the beam directing device along a first axis of beam propagation, so as to cause the primary beam incidence onto the sample along a second axis that is spaced-apart from the first axis, thereby causing the secondary beam propagation towards a region of the detector outside the primary beam hole.
The present invention enables operation in both the xe2x80x9cnormalxe2x80x9d and xe2x80x9ctiltxe2x80x9d operational modes without the need for inclining the sample with respect to the charged particle beam apparatus or vice versa. The term xe2x80x9cnormal modexe2x80x9d used herein signifies the primary beam incidence onto the sample with substantially zero incident angle, i.e., substantially perpendicular to the sample""s surface. The term xe2x80x9ctilt modexe2x80x9d used herein signifies the primary beam incidence onto the sample along an axis forming a certain non-zero angle with the sample""s surface.
Thus, according to one aspect of the present invention, there is provided a method of separating between a primary charged particle beam and a secondary charged particle beam, the secondary charged particle beam resulting from interaction of the primary charged particle beam with the sample, the method comprising:
(a) directing the primary charged particle beam along a first axis passing through an opening in a detector, which has charged particle detecting regions outside said opening;
(b) affecting the trajectory of the primary charged particle beam to provide the primary charged particle beam propagation to the sample along a second axis substantially parallel to and spaced apart from said first axis, thereby causing the secondary charged particle beam propagation to the detecting region of said detector outside said opening.
The present invention, according to its another broad aspect, provides a method of inspecting a sample with a charged particle beam, utilizing the above technique of separating the primary and secondary charged particle beams.
The arrangement is preferably such that the first axis (typically defined by the longitudinal axis of the anode tube) is substantially perpendicular to the sample""s surface. The present invention also provides for operating with a column of the kind having an anode tube (defining the first axis) inclined with respect to the sample""s plane. In this case, the trajectory of the primary charged particle beam is affected to provide the primary charged particle beam propagation along the second axis, which forms an angle with said first axis.
There is thus provided according to yet another aspect of the invention, a method of separating between a primary charged particle beam and a secondary charged particle beam, the secondary charged particle beam resulting from interaction of the primary charged particle beam with the sample, the method comprising:
directing the primary charged particle beam towards a beam directing device along a first axis passing through an opening in a detector, which has charged particle detecting regions outside said opening;
passing the primary charged particle beam through the beam directing device that includes a focusing assembly defining an optical axis forming an angle with said first axis, and a deflection assembly, the deflection assembly being operable to deflect the primary charged particle beam from its propagation along the first axis to the propagation of the primary charged particle beam to the sample along a second axis substantially parallel to the optical axis of the focusing assembly, and to affect the trajectory of the secondary charged particle beam to provide the secondary charged particle beam propagation to the detecting region of said detector outside said opening.
Generally speaking, the second axis of the primary charged particle beam incidence onto the sample intersects the sample at a location spaced apart from a location of intersection between the first axis and the sample.
The beam directing device comprises a focusing assembly (including an objective lens arrangement) that defines an optical axis, and a deflection assembly operable to produce deflection fields that affect the trajectory of each of the primary and secondary beams with respect to the optical axis of the focusing assembly.
In the preferred embodiment of the invention, the first axis of the primary beam propagation towards the beam directing device is substantially parallel to the optical axis of the focusing assembly. This can be implemented by directing the primary beam towards the beam directing device either along the first axis coinciding with the optical axis of the focusing assembly or along the first axis spaced-apart from the optical axis of the focusing assembly. In this case, at least two deflection fields are used to separate between the paths of the primary and secondary beams and ensure the secondary beam propagation to the regions of the detector outside the primary beam hole. The two deflection fields may be pre-lens, pre-lens and in-lens, or in-lens and post-lens (with respect to the deflectors location relative to the objective lens). To eliminate or at least significantly reduce chromatic aberrations of focusing and deflection, either the same two deflection fields, or one or two additional deflection fields can be used.
In another embodiment of the invention, the first axis of the primary beam propagation towards the beam directing device is inclined with respect to the optical axis of the focusing assembly. In this case, the provision of the single deflection field within the beam directing device is sufficient for successful separation between the paths of the primary and secondary beams (i.e., prevent the secondary beam from passing through the primary beam hole of the detector.
It may be desired to incident the primary beam onto the sample with a certain non-zero incident angle (tilt mode). Moreover, the case may be such that the tilt mode is to be applied selectively, namely, to selective locations of the sample, while enabling inspection of this and other locations with the normal mode. To enable application of the tilt mode, the trajectory of the primary beam, as well as that of the secondary beam, is appropriately affected by at least two deflection fields.
There is thus provided, according to yet another broad aspect of the present invention, a beam directing device for use in a charged particle beam apparatus, which defines a primary charged particle beam propagating towards the beam directing device along a first axis and utilizes a detector that is formed with an opening and charged particle detecting regions outside said opening and is accommodated such that said first axis passes through said opening of the detector, the beam directing device comprising:
a focusing assembly that defines an optical axis and is operable to focus the primary charged particle beam onto a sample; and
a deflection assembly operable to affect the trajectory of the primary charged particle beam to direct the primary charged particle beam onto the sample along a second axis substantially parallel to and spaced-apart from said first axis, thereby causing the secondary charged particle beam propagation to the detecting region of said detector outside said opening.
There is also provided according to the invention, a charged particle beam apparatus for inspecting a sample comprising:
an anode tube defining a space of propagation of a primary beam of charged particles, generated by a particles"" source, along a first axis substantially parallel to a longitudinal axis of the anode tube;
a detector formed with an opening and having charged particle detecting regions outside said opening, the detector being accommodated such that said first axis intersects with said opening; and
a beam directing device accommodated in the path of the primary charged particle beam passed through said opening in the detector, the beam directing device comprising a focusing assembly, which defines an optical axis and is operable to focus the primary charged particle beam onto the sample, and a deflection assembly, which is operable to affect the trajectory of the primary charged particle beam to direct the primary charged particle beam onto the sample along a second axis substantially parallel to and spaced-apart from said first axis, and to affect the trajectory of the secondary charged particle beam, thereby providing the secondary charged particle beam propagation to the region of said detector outside said opening.
According a preferred embodiment of the invention, the beam directing device is accommodated such that the optical axis of the focusing assembly is parallel to the longitudinal axis of the anode tube. The deflection assembly comprises at least two deflectors accommodated and operated to sequentially affect the trajectory of the primary charged particle beam at successive regions along the optical axis, and consequently sequentially affect the trajectory of the secondary charged particle beam.
The above can be implemented by substantially coinciding the optical axis of the focusing assembly with the longitudinal axis of the anode tube. In other words, the primary beam enters the beam directing device substantially along the optical axis of the focusing assembly. With this situation, in the normal mode, the primary beam hits the sample at a location spaced-apart from the location of intersection between the optical axis and the sample""s surface. As for the tilt mode, the primary beam impinges onto the sample with a certain angle of incidence so as to hit the sample either at the location in which the optical axis intersects the sample or at the location of the primary beam incidence with the normal mode. Alternatively, the optical axis of the focusing assembly can be spaced-apart from the longitudinal axis of the anode tube. This enables to provide the normal incidence of the primary beam onto the sample substantially along the optical axis and the focusing assembly.
According to another embodiment of the apparatus, the longitudinal axis of the anode tube (defining the first axis) is inclined with respect to the optical axis of the focusing assembly. In this case, the deflection assembly is operable to provide the primary charged particle beam propagation to the sample along the second axis substantially coinciding with the optical axis of the focusing assembly.
The charged particle beam may be an electron beam or a focused ion beam (FIB). The present invention may be used in an SEM or the like tool applied to a specimen, e.g., a semiconductor wafer, for imaging, measurements, metrology, inspection, defect review or the like purposes. For example, the present invention may be used for CD measurements, line profile measurements, copper-interconnects inspection/measurements typically performed after a photolithography process, automatic defect classification, etc.
More specifically, the present invention is used with a SEM system for inspecting wafers, masks or reticles, and is therefore described below with respect to this application.